The present invention relates generally to an improved flue pipe assembly particularly adapted for improving the efficiency of hot water heaters and more specifically to such apparatus which is designed to inhibit heat loss from the hot water within the water heater storage tank of a water heater.
Conventional gas fired water heaters in use today incorporate a center flue pipe which extends upwardly through the water storage tank and which is generally cylindrical in shape. Hot combustion gases from a gas fired burner assembly positioned below the water storage tank are directed upwardly through this center flue pipe which serves to transfer heat therefrom to the stored water surrounding the outer surface of the flue. In many cases the flue will include a device operative to induce turbulence into the flowing hot gases to improve heat transfer to the surrounding water. Such devices are commonly referred to as baffles and typically comprise an elongated zig zag or spiral shaped member suspended in the center of the flue. The combustion gases typically exit the top of the water heater and are directed out of the building within which the water heater is located via a vent or smoke pipe.
When the burner assembly is not firing, ambient air will flow through this flue pipe and cool the heated water in the storage tank thus reducing the overall efficiency of the water heater. In recent years increasing emphasis has been placed on improving the ability of such water heaters to efficiently heat the stored water and to reduce the heat loss therefrom when in a standby mode (i.e. burner assembly in an off condition) to thereby improve the overall operating efficiency.
Various types of insulation have been added to the outer surface of the water storage tank which have greatly reduced heat loss through these outer walls. Additionally, various types of damper arrangements have been incorporated into the external vent pipe to reduce air circulation through the flue pipe during standby periods. However, these damper arrangements generally require a power supply for operating same as well as safety interlocks to insure they are opened before the burner is fired. In some cases these devices may incorporate an arrangement that insures they are in an open position in the event of a power supply failure. Nevertheless, such devices are complex and costly to manufacture and because they require an external power supply, they also result in increased installation costs. Further, gas fired water heater manufacturers generally prefer to avoid the need for an external power supply for operation of their products. Additionally, because these devices are commonly incorporated in the external vent pipe and the internal flue pipe of the water heater is open to the surrounding environment via the vent hood, such dampers are not totally effective in preventing air flow through the flue pipe.
The present invention, however, overcomes the problems associated with the prior art devices by providing an effective insulating barrier between the wall of the flue and any air flow through the flue pipe thereby greatly reducing heat loss when the heater is in the standby mode while still ensuring good effective heat transfer through the flue pipe to the water when the water heater is firing. The primary objective of the present invention is to provide an arrangement which is highly efficient in transferring heat from the combustion gases to the water when the burner is firing but minimizes heat transfer from the water to gases flowing through the flue pipe when the water heater is in a standby mode.
In one embodiment an open cylindrical member of a diameter less than that of the flue pipe is positioned concentrically within the flue pipe and includes a plurality of openings in the sidewalls thereof with radially inwardly extending flanges positioned above the respective openings. A self powered thermally responsive valve assembly operates to selectively open and close the annular space between the inner cylindrical member and the flue pipe at the upper end thereof in response to firing of the burner assembly. Because the center of the cylindrical member is continuously in open communication with the water heater venting system there is no need for costly and complex interlocks with the burner control system. Further, the system does not require any external power supply for operation thus avoiding any increased installation expenses by the end user of the water heater.
In another embodiment, a similar valve assembly is also provided to open and close off the annular space between the cylindrical liner and flue pipe at the lower end thereof to further isolate the air volume in this annular space from convection air currents.
Each of these embodiments offer the advantage of effectively restricting heat loss from the heated water in the tank to air currents flowing through the center of the water heater while in a standby mode and yet also ensures good heat transfer to the tank water from the combustion gases when the burner is firing. For each case the path from burner assembly to the external venting system is continuously open thus eliminating the need for any complicated safety interlocks with the burner controls. Further because the system is self activating, there is no need for any external or auxiliary power supply which would increase installation costs or require periodic maintenance.